It's not a matter of distance for me but heat of the day.
'Bum cream' does nothing, ime, for numb bum but it does prevent sweat rashes, etc. So if it's 20+c outside AND it's a 100km or more ride then I'll apply a good dollop of sudocrem. In cooler weather, I'll not bother with anything regardless of mileage. If my skin is looking red and angry, or feeling saw, after a ride then I use conotrane. If it's not clearing up then I use daktarin.
I haven't used Assos cream though, or that udderly stuff. Maybe that does do something to delay the onset of numb bum... but I'd very much doubt it.